1. Field
The present invention relates to photovoltaic devices and methods of producing such devices. More particularly, the present invention relates to photovoltaic modules that are composed of solar cells having multiple photovoltaic junctions, additional means of converting electrical energy and direct electrical connection of such modules to the electrical utility grid.
2. Related Art
Photovoltaic devices represent one of the major sources of environmentally clean and renewable energy. They are frequently used to convert optical energy into electrical energy. Typically, a photovoltaic device is made of one or more semiconducting material with p-doped and n-doped regions. The conversion efficiency of solar power into electricity of this device is limited to a maximum of about 37%, since photon energy in excess of the semiconductor's bandgap is wasted as heat. A photovoltaic device with multiple semiconductor layers of different bandgaps is more efficient: an optimized two-bandgap photovoltaic device has a maximum solar conversion efficiency of 50%, whereas a three-bandgap photovoltaic device has a maximum solar conversion efficiency of 56%. Realized efficiencies are typically less than theoretical values in all cases.
Multi-layered or multi junction devices are currently manufactured as monolithic wafers, where each semiconductor layer is crystal-grown on top of the previous one. As a result, the semiconductor layers are electrically connected in series and have to be current-matched, in order to obtain maximum conversion efficiency. This current-matching procedure complicates the design and decreases the efficiency of the device. The latter becomes particularly evident when considering the effect of spectral filtering on the device efficiency. If a part of the solar spectrum is absorbed or scattered, e.g. by water vapors, the resulting disproportional decrease of photocurrent in one of junctions will limit the current through the whole device and thus decrease its conversion efficiency.